The present invention relates to pressure-measuring apparatus in general and in particular to an apparatus and method for measuring the internal pressure of an enclosed member, such as the intra-ocular pressure of an eye.
The intra-ocular pressure of an eye is that pressure associated with the presence of a fluid in the eye called aqueous humour, which is known to completely fill the anterior and posterior chambers of an eye. It is also known that it flows through the chambers in fluid passageways. If, for any reason, such as the presence of a condition known as glaucoma, the fluid flow of the aqueous humour through the eye is interrupted or impaired, a change in the intra-ocular pressure of the eye will result. It is, therefore, quite important to be able to readily, accurately and safely measure intra-ocular pressure in order to diagnose glaucoma and the like.
However, because the eye is an enclosed member, it is not practical to measure the intra-ocular pressure in a living eye directly. Accordingly there has been developed a variety of instruments for measuring the pressure indirectly. The instruments used for this purpose are known as tonometers.
In general, most tonometers in use today work on either of two principles. The first involves applying a known force to the cornea of an eye and measuring the deformation produced. The instruments using this principle are known as "impression or indentation" tonometers. The second involves applying a standard or known deformation to the eye and measuring the force required to produce the deformation. The instruments using the second principle are called applanation tonometers. The two principal types are distinguished in that in the first case, the cornea is indented and in the second case, the cornea is merely flattened. The present invention utilizes the second principle.
Because the eye is a relatively sensitive organ, it is difficult for one whose eye is being examined to restrain from flinching or blinking when the cornea is touched. For this reason, the use of many prior known tonometers requires that the eye be anesthetized. Since anesthetizing an eye is considered a serious medical procedure, trained personnel are frequently required to carry it out. Even when trained personnel are available, the anesthetizing of an eye is to be avoided if at all possible because of the possibility of an inadvertent injury to the eye as might be caused by rubbing or striking the cornea with a sharp object which goes unnoticed because of the anesthetic. If the use of an anesthetic can be avoided, not only will the possibility of an inadvertent injury going unnoticed be reduced, but intra-ocular pressure measurements could be taken by a wider group of personnel at lower cost and with the attendant advantage that more people would undoubtedly be able to afford an earlier diagnosis of a serious abnormal eye condition.
In addition to the problems associated with having to anesthetize an eye when using many of the prior known tonometers, there has been also a problem in interpreting the results of the measurements taken. The results of the measurements taken with many prior known tonometers are recorded on paper by a pen- recorder. The wave forms recorded are irregular and vary from patient to patient. This is due in part to the fact that the volume of the eye is invariably changed during a measurement. Because of this, it is the practice to make allowances for the pressure change due to the change in eye volume and to also allow for the effects of the various mechanical forces involved in the bending of the corneal surface. These factors tend to result in inaccurate measurements and require a good deal of guessing, which is especially difficult using pen-recorded pressures, as to what the actual equivalent pressure is in an undisturbed eye.
In addition to the desirability of providing a tonometer apparatus which does not require the anesthetizing of an eye and which provides a more accurate readout of intra-ocular pressure without the necessity for relying on the educated interpretations of pen-recorded pressures, there is also a need for a reliable and accurate means for measuring the internal pressure of other types of enclosed members. For example, a considerable need exists for an apparatus for reliably, accurately and automatically measuring tire pressure on large trucks, aircraft and other vehicles.
Presently, conventional means for accurately measuring tire pressure involves the use of a gauge applied to the valve stem of the tire. This results in a direct measurement of pressure. For vehicles carrying a number of tires, this procedure is time-consuming and, therefore, costly.
There is also believed to be a considerable need for analyzing the condition of fruits and vegetables in a more automatic and economical fashion. Since the condition of the skin of fruits and vegetables is often closely related to the condition of the produce itself, it is believed that an accurate measurement of the mechanical characteristics of the skin as well as the internal pressures in the produce can quickly determine the condition of the produce.